Depending on the type of HVACair conditioning or heat pump system it is and the efficiency range of the system will depend on the type of metering device the system has installed by the HVAC manufacturer of the air conditioner or heat pump system. Lower efficiency HVAC air conditioner and heat pump models have fixed orifice types while higher efficiency systems have thermostatic expansion valves installed in the HVAC air conditioner or heat pump system. The thermostatic expansion valve is far more complex than the fixed orifice metering device as the thermostatic expansion valve metering device modulates the refrigerant flow based on the temperature of the refrigerant temperature in the evaporator coil. This allows a specific amount of refrigerant, based on demand, to be metered into the evaporator coil while the fixed orifice metering device allows the same amount of refrigerant to enter the enter the coil no matter the conditions or the demand. As the liquid refrigerant enters the metering device it changes temperature and pressure. A partial amount of the liquid refrigerant flashes into a refrigerant gas or vapor as it leaves the metering device and enters the evaporator coil.
Thermostatic Expansion Valve – Also referred to as the TEV or TXV for short the thermostatic expansion is used in many air conditioning and heat pump applications including use in chillers for chilled water systems. TXVs respond to the temperature of the refrigerant leaving the evaporator coil or evaporator barrel. The TXV has a sensing bulb that holds a slight refrigerant charge inside the bulb. The TXV bulb is remote from the TXV and is attached to the TXV via a capillary tube or cap tube. As the temperature increases and decreases the refrigerant inside the bulb responds by expanding and contracting based on the temperature pressure relationship of refrigerants. As the refrigerant expands and contracts causes a bellows to move in and out which causes a piston to open and close precisely based on the leaving temperature of the refrigerant in the evaporator coil. This allows the TXV metering device to feed the evaporator coil the precise amount of refrigerant it needs to maintain a specific superheat. Because the TXV precisely meters the refrigerant the TXV is used in many air conditioner and heat pump systems that need a higher efficiency. A properly engineered and installed system that uses a TXV will only give the evaporator coil what it demands and nothing more or nothing less.
The other type of expansion valve used in HVAC is the fixed orifice. The fixed orifice is simple and can be either a piston type or utilize a distributor that feeds capillary tubes which terminate in the evaporator coil. The piston type comes in various sizes and is interchangeable with the capacity or tonnage of the condensing unit. Always follow the manufacturers instructions for piston size if using the fixed orifice for HVAC refrigeration.
Capillary tubes (cap tubes) are also used to meter refrigerant to the evaporator coil. These are considered fixed orifice since they hole in the tube(s) is a fixed size.
There are other non-conventional types of metering devices used for HVAC refrigeration including the float type and the electronic type however these are not commonly used in standard HVAC equipment.
There are other types of metering devices used in HVAC vapor compression refrigeration but these are the most common types are used in HVACR in both the commercial market and residential markets. As systems grow more sophisticated and complex that is likely to change. Electronic types are the next big step in refrigeration used for air conditioning and heat pump systems.
Evaporator Coil – the evaporator coil is responsible for absorbing heat into the coils and the refrigerant. The evaporator coil is an essential component of vapor compression refrigeration. This is usually done as result of passing air or water over the evaporator coil where a heat exchange process takes place. Heat leaves the air or water (whatever medium is used) and is absorbed into the coil and the liquid refrigerant. As more and more heat is absorbed by the evaporator coil and the refrigerant again changes state from a liquid to a refrigerant vapor. By the time the refrigerant leaves the evaporator coil the refrigerant should be all vapor and ready to be received again by the refrigeration compressor.
A new evaporator coil replacing an old leaking coil
The evaporator coil is typically inside the air handling unit or it can be installed in the duct work near the air handling unit. When the air handler blower turns on and the condenser is running refrigerant flows from the compressor through the condenser coils and then through the metering device. and into the evaporator coil. The refrigerant changes state in the evaporator coil and again in the condensing unit. In the evaporator coil the change of state is from a liquid to a vapor while in the condenser the change of state is from a vapor to a liquid. This is all necessary for the process of refrigeration to work properly. The typical evaporator coil is constructed of copper with aluminum fins surrounding the copper coils. The aluminum adds surface area to the copper coils and enhances the heat exchange rate making the coils more efficient.
Components of Vapor Compression Refrigeration – The Evaporator Coil
The condenser coil receives the high pressure high temperature refrigerant vapor from the compressor and immediately begins to remove heat from the refrigerant vapor. As the refrigerant vapor makes its way through the condenser coil more and more heat is removed and the refrigerant vapor changes state from a refrigerant vapor to a refrigerant liquid. While the liquid refrigerant changes temperatures from a higher temperature to a slightly lower temperature the pressure remains constant. As the refrigerant vapor leaves the condenser coil it makes it way to the metering device.
Most condensing units utilized in HVAC whether it is the commercial HVAC field or the residential HVAC field are constructed of copper with aluminum fins. The aluminum fins are mechanically attach to the copper tubes that make up the condenser coils or the piping that channels the refrigerant from the beginning of the condenser coils all the way to the end or exit of the condensing unit where the refrigerant makes its way to the metering device. The aluminum fins attached to the copper pipe enhance the heat exchange process in the air source condensing units. A few manufacturers use only aluminum coils in their condensing units to take advantage of the efficient heat exchange properties of aluminum. The aluminum coils also have fins on them and these are usually referred to as spine fin coils. The spine fins extend the surface area of the aluminum coil enhancing the heat exchange process for efficiency purposes.
Many condensing units also house the compressor and condensing unit controls. The condensing typically utilizes its own dedicated circuit from the main circuit breaker panel. In a split system the condensing unit will have a line set or refrigeration lines that run from the condensing unit to the evaporator coil inside the building. The condenser is an essential component of vapor compression refrigeration.
Components of Vapor Compression Refrigeration – The Condenser
Centrifugal compressors are used in many applications including pipeline transport of natural gas along with many other related applications including vapor compression refrigeration. Centrifugal compressors have different operating limitations than reciprocating compressors, scroll compressors, and rotary compressors. The centrifugal compressor has an operating range where it operates efficiently however outside of this range, either the over or under that operating, the centrifugal compressor is basically useless. Other issues with centrifugal compressors make them not ideal for use in certain compressor applications because of the operating range issues. Continue reading “HVAC Centrifugal Compressors” »
The rotary compressor used in HVAC applications for air conditioning and heating systems offer some of the most efficient air conditioner and heat pump systems one can find on the market. Despite the rotary compressor being a key component of an air source air conditioner or heat pump the efficiency offered by the rotary compressor when used as a modulating compressor can exceed efficiency ratings of some geothermal systems. When the rotary compressor is coupled with an inverter and the proper inverter control system the efficiency of the air source air conditioner or heat pump system spike to match those of some geothermal air conditioner or heat pump systems.
This chilled water unit is a new ammonia chiller. This unit is also unique in that it has no compressor. Instead it uses the chemical properties of ammonia and heat from natural gas to condense the refrigerant which is ammonia. A chiller provides chilled water to a chilled water loop. Cold water is circulated through a loop where it is direct into a coil in an air handler. Air is blown across a coil where a heat exchange process takes place. The air handler discharges cool conditioned air throughout a duct system. Chilled water systems are typically used in commercial air conditioning applicationsbut there are some chilled water systems in residential applications. Continue reading “Ammonia Chilled Water System” »
The process of refrigeration uses the vapor-compression refrigeration cycle to move heat from one place to another. In many HVAC air conditioning and heat pump applications the process of refrigeration or the vapor-compression cycle is what makes it work. This process, in the air conditioning mode, moves heat from inside the structure to the outside. Heat pumps draw heat from the outside even when it is cold and move the heat inside using the vapor-compression cycle or refrigeration. Heats pumps by the way, heat in the winter and cool in the summer but it uses the refrigeration process to accomplish the heating and cooling. The process of refrigeration or vapor-compression cycle in HVAC air conditioning and heat pump applications is a mechanical and chemical process of moving heat from one place where it is not wanted or needed and moving it to another place. Continue reading “Refrigeration HVAC – Vapor Compression Refrigeration” »
Most data centers do not utilize HVAC DX (Direct Expansion)CRAC units or air conditioning but instead use chilled water systems to provide cooling for the data center. In smaller buildings such as remote telecom buildings DX air conditioning systems are used to keep the equipment cool and operating efficiently. A DX CRAC Unit uses vapor compression refrigeration to provide cooling with a chemical refrigerant versus a chilled water system that utilizes chilled water from a chiller. CRAC Units or Computer Room Air Conditioners also serve telecom facilities for telephone services including cellular telephone facilities in remote locations. The CRAC units that serve telecom facilities in remote locations are typically DX CRAC Units that have utility back-up by stand-by generator so that if the utility fails to provide electricity for whatever reason the stand-by generator kicks on and provides power so that the telecom facility can remain on line. Continue reading “DX CRAC Unit Condensers” »